Text-to-speech (TTS) systems convert written text to sound. This can be useful to assist users of digital text media with a visual impairment, dyslexia and other reading impairments, by synthesizing speech representing text displayed on a computer screen. Speech recognition systems have also progressed to the point where humans can interact with and control computing devices by voice. TTS and speech recognition combined with natural language understanding processing techniques enable speech-based user control and output of a computing device to perform tasks based on the user's spoken commands. The combination of speech recognition and natural language understanding processing is referred to herein as speech processing. Such TTS and speech processing may be used by computers, hand-held devices, telephone computer systems, kiosks, and a wide variety of other devices to improve human-computer interactions.